Nasdaq-100 and QQQ
The Nasdaq-100 tracks 100 of the largest non-financial companies listed on Nasdaq; QQQ is an ETF that holds and tracks that index.
The Nasdaq-100 is a modified market-capitalization-weighted index of the largest non-financial companies on the Nasdaq Stock Market. It is reconstituted each year and rebalanced periodically to limit the weight of the largest holdings. QQQ is an exchange-traded fund that holds the index's constituents and tracks its performance, less fees.
Concentration matters. The largest holdings can account for a large share of the index, so single-stock moves can drive index performance. Traders use QQQ for large-cap growth and technology exposure, and its weights shift with index changes and with price moves.
Worked example (illustrative numbers, not real data): a stock with a 10 percent index weight rises 1 percent. The index rises by about 0.1 percent from that stock, all else equal.